1. Field of the Invention
The present invention relates to a method of manufacturing a photochromic molded article through photopolymerization of a photopolymerizable composition comprising at least one photopolymerizable monomer, at least one photoinitiator and at least one photochromic compound, and in particular to such a method which allows fast photopolymerization of the composition without degrading the photochromic compound.
2. Description of the Prior Art
Photopolymerizable compositions and in particular photopolymerizable compositions for making ophthalmic lenses are usually molded by filling the cavity of a two-part mold made of transparent mineral glass with a liquid photopolymerizable monomer composition and by irradiating the composition with an appropriate UV-light for polymerizing the composition.
A mold and an apparatus typically used for molding ophthalmic lenses are disclosed in U.S. Pat. Nos. 5,547,618 and 5,662,839.
Only one lens at a time can be molded using such a mold and apparatus and therefore the lens manufacturing efficiency is highly depending upon the duration of the irradiation polymerization step, at least up to a degree of polymerization at which the composition is sufficiently cured to become self-supporting or to be safely handled.
Document U.S. Pat. No. 5,621,017 discloses a method for producing photochromic cured products which comprises irradiating a photopolymerizable monomer composition including a photopolymerizable monomer, a photoinitiator and a photochromic compound. The amount of photochromic compound that can be incorporated in the composition must be kept in a very narrow range of 0.001 to 0.2 parts by weight and, preferably, of not smaller than 0.01 part by weight but smaller than 0.1 part by weight with respect to 100 parts by weight of the polymerizable monomer.
When the amount of photochromic compound is not larger than 0.001 parts by weight, the photochromic property is not obtained with sufficient degree of color concentration.
When the amount of the photochromic compound is larger than 0.2 parts by weight, on the other hand, the polymerization with light is not completed, in a short period of time. When the polymerizable monomer is not polymerized to a sufficient degree, photochromic property tends to be deteriorated within a short period of time.
Preferably, the photoinitiator used in the compositions of U.S. Pat. No. 5,621,017 exhibits a main absorption in an ultraviolet region and a molar absorption coefficient at 400 nm of larger than 150 lit./mol.cm, i.e. the photoinitiator has preferably a sufficient absorption in the visible region.
This method has several disadvantages.
During photopolymerization, the photochromic compound is submitted to intense UV radiation, which according to the prior art and in particular to U.S. Pat. No. 5,910,516, has two consequences:
The darkening of the photochromic compound which then filters UV-visible rays consequently limits the effectiveness of the photoinitiator activation. Therefore, the time necessary for polymerizing and achieving the final degree of polymerization or a degree of polymerization sufficient for a safe handling of the molded article is lengthened.
Prolonged irradiation for completing polymerization increases the risk of degrading the photochromic compound.
In order to solve the above technical problems, U.S. Pat. No. 5,910,516 describes the use of a UV filter capable of eliminating ultraviolet light having a wavelength below 400 nm. Elimination of the shorter UV light from the irradiation beam does effectively suppress or reduce the color development of the photochromic compound.
Although this method slightly increases the polymerization rate, as the photoinitiator can be activated by UV-visible radiations having wavelengths around 400 nm, the major part of the UV irradiation does not reach the photoinitiator and consequently cannot activate it.
Thus, there is still a need to provide a faster UV polymerization method for the molding of photochromic articles and especially for compositions including higher amounts of photochromic compounds and which leads to ophthalmic lenses having excellent photochromic performance (high colorability, high speed of darkening and lightening, resistance to photodegradation, and good durability of photochromic properties).
The object of the present invention is achieved by providing a method of manufacturing a photochromic molded article, such as an ophtalmic lens, comprising the steps of:
(a) filling a mold with a photopolymerizable monomer composition containing:
at least one photopolymerizable monomer;
at least one photoinitiator exhibiting a main absorption in a UV region; and
at least one photochromic compound capable of coloring upon UV irradiation;
(b) photopolymerizing said photopolymerizable composition under irradiation with a light comprising a UV portion and UV-visible portion; wherein,
before the beginning of the photopolymerization step, said photopolymerizable composition is subjected to a pre-heating step in which the temperature of said photopolymerizable composition is increased to a temperature reducing or preventing coloration of said at least one photochromic compound during the subsequent photopolymerization step.
The pre-heating step comprises heating the photopolymerizable composition at a temperature above the ambient temperature (ambient temperature meaning xe2x89xa625xc2x0 C.) at which the photochromic compound is predominantly in a non-excited state or uncolored form even at the beginning of the irradiation by the UV light. Usually, the pre-heating comprises heating the photopolymerizable composition to a temperature ranging from 30 to 90xc2x0 C., preferably from 40 to 60xc2x0 C. and typically around 50xc2x0 C.
The pre-heating step can be performed using any classical means such as air oven heating, hot-water heating, infra-red radiation heating and microwave heating.
The photopolymerizable monomer composition can be pre-heated while in the mold cavity, before beginning photopolymerization, or it can be pre-heated before being poured in the mold cavity. In any case, the photopolymerizable composition must be at a temperature higher than ambient temperature (xe2x89xa625xc2x0 C.) and sufficient to at least reduce, and preferably prevent photochromic compound coloration during the photopolymerization step.
The photopolymerization step of the process of the invention is classical and generally comprises irradiating the photopolymerizable composition with a light comprising UV radiations. Preferably, the irradiation light also comprises a UV-visible portion having a wavelength spectrum around 400 nm.
Preferably also, the photopolymerization step comprises a pre-polymerization step in which the composition is irradiated with a UV light, preferably including a UV-visible portion around 400 nm, at a first intensity, followed by a further polymerization step with a UV light, preferably also including a UV-visible portion around 400 nm, at a second intensity higher than the first intensity.
By a UV-visible light portion around 400 nm, it is intended a light having a wavelength spectrum of 380 to 450 nm.
Usually, the UV light has a wavelength spectrum of 250 to 400 nm.
The pre-polymerization step, which is preferably of a short duration, generally will allow for obtaining a sufficient polymerization degree of the composition so that the pre-polymerized article is self-supporting and can be safely handled.
Generally, the pre-polymerization step can last from 1 second to 10 minutes and preferably 5 seconds to 1 minute.
The photopolymerizable monomers of the composition of the present invention may be any known monomer having a radical polymerizing group such as, for example, acrylate group, methacrylate group, vinyl group, and the like. Mixtures of such radical polymerizing monomers may be used.
A preferred class of radical polymerizing monomers are monomers of formula (I): 
wherein
R1 and R2, independently of each other, may represent hydrogen or a lower C1-C6 alkyl radical, X represents
xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94CH2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, or xe2x80x94C(CH3)2xe2x80x94,
m and n are integers, and the sum m+n has an average value ranging from 0 to 40, preferably 0 to 10, more preferably 2 to 7.
Particularly, preferred monomers having formula (I) are those wherein R1 is CH3 and R2 is hydrogen and X represents xe2x80x94C(CH3)2xe2x80x94 and m+n has an average value of 2 to 7, and in particular m+n is 2 to 4.
A particularly preferred monomer of formula (I) is 2,2-bis(4-methacryloxy diethoxy phenyl) propane.
Preferably, the monomer of formula (I) represents at least 50% by weight and preferably 70 to 100% by weight based on the total weight of the polymerizable monomers present in the composition.
Among other radical polymerizable monomers that can be used in the present invention, there may be cited: a vinylbenzyl compound represented by the general formula (II) 
wherein Rxe2x80x21, Rxe2x80x22, Rxe2x80x23 and Rxe2x80x24 may be the same or different and are halogen atoms, X1, X2 and X3 are oxygen atoms or sulfur atoms, j, k and m are 0 or 1, respectively, and j=0 when k=0 or k=j=0 when m=0, but X1, X2 and X3 are not simultaneously sulfur atoms when m=k=j=1.
Specific examples of the radical polymerizable monomer that can be favorably used in the present invention include the following compounds: diacrylate compounds or dimethacrylate compounds, such as diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, butanediol dimethacrylate, hexamethylene dimethacrylate, 2,2-bis(4-methacryloyloxyethoxy-3,5-dibromophenyl) propane, and 2,2-bis(4-methacryloyloxyethoxyphenyl)propane; acrylate compounds or methacrylate compounds having an epoxy group, such as glycidyl acrylate, glycidyl methacrylate, beta-methylglycidyl acrylate, beta-methylglycidyl methacrylate and bisphenol A-monoglycidyl ether methacrylate; and vinylbenzyl compounds, such as bis-4-vinylbenzyl ether, bis-4-vinylbenzyl sulfide, 1;2-(p-vinylbenzyloxy)ethane, 1,2-(p-vinylbenzylthio) ethane, bis-(p-vinylbenzyloxyethyl)sulfide, etc., together with other radical polymerizable monomers that are copolymerizable therewith.
Examples of other radical polymerizable monomers that can be used with the above mentioned monomers, and in particular monomers of formula (I), include unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic anhydride, and fumaric acid; acrylic and methacrylic ester compounds such as methyl acrylate, methyl methacrylate, benzyl methacrylate, phenyl methacrylate, tribromophenyl methacrylate, 2-hydroxyethyl methacrylate, bisphenol-A dimethacrylate, trifluoromethyl methacrylate, urethane acrylate and epoxy acrylate; fumaric ester compounds such as monomethyl fumarate, diethyl fumarate and diphenyl fumarate; allyl compounds such as diallyl phthalate, diallyl terephthalate, diallyl isophthalate, diallyl epoxysuccinate, diallyl maleate, allyl cinnamate, allyl isocyanate, diallyl chlorendate, diallyl hexaphthalate, diallyl carbonate, and allyl diglycol carbonate; and aromatic vinyl compounds such as styrene, chlorostyrene, alpha-methylstyrene, alpha-methylstyrene dimer, vinyl naphthalene, isopropenyl naphthalene, bromostyrene and divinylbenzene. These monomers may be used in a single kind or being mixed together in two or more kinds.
Another class of radical polymerizable monomers which can be used preferably with monomers of formula (I) are bisphenol-A poly (alcoxylate) dimxc3xa9thacrylate such as bisphxc3xa9nol-A 30 ethoxylate dimxc3xa9thacrylate.
For the purpose of improving the performance of the photochromic compound(s), it may be desirable to add plastifying agents and especially aromatic ring containing inert plasticizers.
Such kind of plasticizers are described in WO patent application WO 95/10790 and are incorporated herein by reference.
A preferred plasticizer is a poly(ethyleneglycol) of molecular weight 200 benzoate-terminated at both ends.
As the photopolymerization initiator, any widely known compound can be used without limitation that is added for photopolymerizing the radical polymerizable monomers. Among the photopolymerization initiators that can be suitably used in the present invention, it is desired to use an acetophenone photopolymerization initiator, an xcex1-dicarbonyl photopolymerization initiator, an acylphosphine oxide photopolymerization initiator and a bisacylphosphine oxide photopolymerization initiator from the standpoint of favorably carrying out the photopolymerization and obtaining the polymer with no color. More specifically speaking, it is desired to use the compounds represented by the following formulas (III), (IV) and (V) 
wherein R3 and R4 are alkyl groups which together may form a cyclohexane ring, and R5 is an alkyl group or a hydrogen atom, 
wherein R6 is the same or different and is a methyl group, a methoxy group or a chlorine atom, e is 2 or 3, and R7 is phenyl group or methoxy group, 
Examples of photopolymerization initiators that can be preferably used in the present invention are as described below:
Acetophenone Polymerization Initiators
1) 1-Phenyl-2-hydroxy-2-methylpropane-1-one,
2) 1-Hydroxycyclohexylphenyl ketone, and
3) 1-(4-Isopropylphenyl)-2-hydroxy-2-methylpropane-1-one.
xcex1-Dicarbonyl Compounds
1) 1,2-Diphenylethanedione, and
2) Methylphenylglyoxylate.
Acylphosphine Oxide Photopolymerization Initiators
1) 2,6-Dimethylbenzoyldiphenylphosphine oxide,
2) 2,4,6-Trimethylbenzoyldiphenylphosphine oxide,
3) Methyl 2,4,6-trimethylbenzoyldiphenylphosphinate ester,
4) 2,6-Dichlorobenzoyldiphenylphosphine oxide, and
5) 2,6-Dimethoxybenzoyldiphenylphosphine oxide.
These photopolymerization initiators can be used in a single kind or in a combination of two or more kinds.
Bisacylphosphine Oxide Photo Polymerization Initiators
1) Bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.
Among the above-mentioned photopolymerization initiators, those that have a main absorption in the ultraviolet region and have a molar absorption coefficient at 400 nm of not smaller than 150 lit./mol-cm can be effective even by using visible light and are desired since they deteriorate photochromic compounds little during the polymerization.
Specific examples include:
1) 2,4,6-Trimethylbenzoylphenylphosphine oxide, and
2) Bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.
The preferred photo-initiators are the following photo-initiators commercialized by CIBA-GEIGY: 
In the present invention, the photo-initiator is added in usual amounts, namely from 0.005 to 1 part by weight and, preferably, from 0.05 to 1 part by weight per 100 part by weight of polymerizable monomers.
Preferably, the composition of the present invention does not contain any thermal initiator.
If some thermal initiators are used, which is not recommended, one may choose thermal initiators forming radicals at a temperature higher than the temperature of the preheating step.
Some specific organic peroxides that are stable at moderate temperature can be used, especially 00-t-amyl-0-(2-ethylhexyl)-monoperoxycarbonate, 00-t-butyl-0-(2-ethylhexyl)-monoperoxycarbonate. The half lives of these two initiators are higher than 100,000 hours at 50xc2x0 C. meaning that very few radicals are formed at 50xc2x0 C. to sufficiently result in polymerization during the preheating step.
The photochromic compound of the composition of the invention can be any known organic photochromic compound including those used in the optical field.
The preferred photochromic compounds are spirooxazine, chromene and fulgides compounds. The chromene compounds and especialy naphtopyrans are most preferred.
Photochromic spirooxazine compounds are compounds which are well known in the art and are disclosed, inter alia, in patents U.S. Pat. No. 5,139,707 and U.S. Pat. No. 5,114,621 (spiro(indoline-quinazolinoxazine) and (spiro(indoline-benzothiazolooxazine)), EP-A-0,245,020 (spiro[indoline-[2, 3xe2x80x2]-benzoxazine]), JP-A-03251587 (spiro[indoline-[2, 3xe2x80x2]-benzoxazine] substituted at the 6xe2x80x2 position) and WO-96/04590 (spiro[indoline-[2, 3xe2x80x2]-benzoxazine] having a cyano or phenylsulphonyl group at the 6xe2x80x2 position).
Chromenes are also well known photochromic compounds. These compounds are disclosed, inter alia, in patents U.S. Pat. No. 5,066,818, WO-92/09593, EP-A-0,401,958, EP-A-0,562,915 and WO-93/17071.
The compositions of the invention can include a single photochromic compound or a mixture of two or more photochromic compounds.
The amount of photochromic compounds that can be used in the photopolymerizable compositions may range from 0.001 to 1.0 parts by weight, preferably 0.05 to 0.5 parts by weight, per 100 parts by weight of the photopolymerizable monomers.
Using the process of the present invention, even with compositions including more than 0.2 parts by weight of photochromic compound, there is obtained a fast polymerization without degradation of the photochromic properties.
The photopolymerizable composition can contain, as necessary, in the usual amounts, various stabilizers and additives such as mold-releasing agent, an ultraviolet absorber, an ultraviolet stabilizer, an antioxydant, a coloring inhibitor, an anti-yellowing agent, a whitening agent, an antistatic agent, a fluorescent dye, a dye, a pigment, a perfume or the like.
According to the manufacturing process of the invention, there can be produced in a short time a photochromic cured product which is free from deterioration of the photochromic compound and which exhibits photochromic performances, in particular photochromic spectrokinetic performances, equal or even better than prior art photochromic products. Therefore, the cured product is useful as an organic lens having photochromism, in particular a photochromic opthalmic lens.